Displacement follower



June 21, 1960 cs. A. HOTHAM DISPLACEMENT FOLLOWER 2 Sheets-Sheet 1 Filed June 14, 1957 IIII X IIIIIIRU WIIIII.

June 21, 1960 G. A. HOTHAM 2,942,121

DISPLACEMENT FOLLOWER Filed June 14, 1957 2 Sheets-Sheet 2 asa 'r-ns g A Hoff/4A4,

United States Patent'YOflice.

Patented June .21, 19 60 DISPLACEMENT FOLLOWER Geoffrey A. Hotham, 1383 E. Valley Road, Santa Barbara, Calif.

Filed June 14, 1957, Ser. No. 665,708

9 Claims. (Cl. 250-217) a This invention relates to improvements in displacement followers of the type in which a light spot is projected to a part of a moving object and in which the light spot is moved so as to be substantially continuously directed to a part of the object while the object is moving, and in which the movement of the object is detected and measured or otherwise indicated or utilized by measuring the displacement of the light spot.

One of the objects of this invention is to provide an improved arrangement in which light reflected from the moving object is detected.

Another object of the invention is to provide in such a system that employs reflected light, an improved arrangement of light projecting means and light detecting means which provide a relatively high sensitivity to reflected light and a relatively low sensitivity to extraneous light.

Another object of the invention is to provide an improved arrangement for adjustably setting the sensitivity of such a displacement follower. I

Another object of the invention is to provide an arrangement in which the signal-to-noise ratio of the reflected light is increased.

The foregoingand other objects of the invention will be understood by reference to the following description of a single embodiment which is illustrated companying drawing, wherein: Y

Figure l is a perspective view of my improved displacement follower;

Fig. 2 is a schematic diagram of my improved displace ment follower; j

Fig. 3 is an enlarged diagram of the displacement follower with some parts broken away;

, Fig. 4 is a fragmentary view of part of the focusing mechanism taken on the line 4--4 of Fig. 3;

Fig. 5 is a fragmentary vertical section taken on the line 55 of Fig. 3; and

Fig. 6 is a cross-sectional view taken on the line 6-6 of Fig. 3.

In the embodiment of the invention illustrated in the drawings, the displacement follower comprises a housing 10 which includes optical means for projecting a small spot X of light onto an edge or other discrete part of a moving object 12, and means including a photo-multiplier tube 14 for receiving light reflected from the object, together with means controlled by the photo-multiplier for moving the spot X of light in such a way that its image projected toward the. object is continuously focused on the same part of the object. Except for the means that projects the spot of light, the housing is substantially light-tight.

One of the novel features of this invention involves the arrangement by which the sharpness of the image so formed on the object may be readily adjusted. other feature resides in the arrangement for ad usting the sensitivity of the apparatus. Another feature involves the orientation of the photo-multiplier tube" and in the ee- 2 the light projecting system in order to minimize the effects of extraneous signals. And still another feature of the invention involves an arrangement for reducing noise otherwise associated with the light source.

The housing 10 is of substantially rectangular configuration and is provided with support legs 15 hearing rubber feet 16 projecting downwardly from the lower surface of the elongated base member 18.. Front and rear walls 20 and 22 are respectively rigidly mounted at opposite ends of the base member 18, and intermediate apertured wall member 24 is also rigidly secured in an upright position on the base member 18. A chassis 26 hearing electronic equipment including amplifiers 62 and 64, is firmly attached to the rear wall 22 and the intermediate wall 24 near the upper part of the housing 10. A U-shaped cover member 27 comprising side walls 28 and 30 and an upper wall 32 serves to complete the housing. The cover member 27 is firmly but removably secured to the base member 18 and to the end walls 22 and 20 by suitable fasteners (not shown). A handle 34 is firmly fastened to the upper part of the cover 27 to facilitate carrying the displacement follower.

The spot X of light is provided by a cathode ray 0scil-.

tioned in preestablished positions therealong. By virtue of this arrangement, the spacing between the face 42 of the cathode ray oscilloscope tube may be adjusted relative to the front wall 20 and hence the lens 56.

The oscilloscope tube 40 is provided with an electrongun 44 which is arranged in a conventional manner to cause a beam of accelerated electrons 46 to be projected toward the face 42 of the tube. In practice, a Type 3 ACPlS A tube has been employed. Such a tube bears a screen in the form of a coating 43 composed of a P15 phosphor on the interior surface of the tube face 42. When the beam 46 strikes that screen, the particular part of the screen so struck becomes excited, thereby emitting radiation, and thus providing a spot X of light. Suitable means, such as a pair of vertical deflection plates 48, are employed to deflect the beam along a vertical line Z-Z that is. transverse to the longitudinal axis Y'--Y and thus to move the line Z-Z.

Two tubular members 50 and 52 are mounted on thefront Wall 20 and project therefrom. The axis 51 of the first tubular member 50 coincides with the axis Y-Y of 1 the cathode ray tube 40. The axis 53 of the second tubular member 52 is inclined downwardly toward the longitudinal axis Y-Y, converging therewith at a point a few inches in front of the housing. In practice, the.

two tubes have about the same length, and their outer ends are about equally spaced from the front wall 20.

A lens 56 of rather short focal length is mounted at the l outer end of the first tubular member 50. The lens 56 serve to focus an image of the light spot X at a point at the front end of the housing.

is sharply focused on a part of the object which is characterized by a line of demarcation that separates an or nearly horizontal. This line of demarcation may be;

provided by an extreme edge of the object.

spot X along the vertical In practice, the image 1 is formed at the position of the moving object 12 and- A photo-multiplier tube 14 is mounted in the second tubular member 52. The photo-multiplier tube is of the end-on type, in which a photosensitive surface 60 is arranged at one end thereof. In the displacement follower of this invention the photo-multiplier tube 14 is mounted in the tubular member 52, with the photosensitive surface 60 at the outer extremity thereof and with the plane of the photosensitive surface 60 substantially normal to the axis 53 of the tubular member 52. With this arrangement, the photosensitive surface looks at the part of the object 12 upon which the image of the light spot X is focused. By virtue of the fact that the photosensitive surface is thus inclined to the longitudinal axis YY, the photosensitive surface 60 has maximum sensitivity to light from an object located near the intersection of the axes 51 and 53 of the two tubular members, and also reduced sensitivity to extraneous light that may reach the photosensitive surface 60 from other directions.

In operation, the output of the phototube 14 is ap lied to the input of an amplifier 62, thus producing at the output of the amplifier a signal which changes in magnitude in accordance with the change of intensity of light striking the photosensitive surface 60. In practice, the displacement follower is mounted in a room of low am bient illumination or is covered by a blackout cloth so that substantially the only radiation striking the photosensitive surface 60 is that which is reflected thereto from the part of the object 12 upon which the image of the light spot X is focused. Thus, with the light spot X in a fixed position, in which the image is focused at the upper edge of the object 12, then if the object moves down, the intensity of illumination striking the photosensitive surface 60 diminishes, thus reducing the voltage at the output of the amplifier 62. The output of this amplifier 62 is impressed upon the vertical deflecting plates 48. The polarity of the voltage thus applied to the deflecting plates 48 is so chosen that the electric field created between the deflecting plates causes the beam 46 to be deflected in such a direction, in this case upwardly, so that the image of the light spot X moves in a corresponding direction, in this case downwardly, thus following the edge of the object 12. By employing an amplifier 62 of high gain, the intensity of the illumination striking the photosensitive surface 60 tends to be maintained very nearly constant. For this reason, the light spot X moves up and down on the surface 42 of the cathode ray tube 40 in a manner which corresponds to the movement of the object 12 up and down.

The output of the amplifier 62 is also applied to a cathode follower 64 which feeds an external signal indicator 68, such as an indicating or recording galvanometer. As in any negative feedback system, the intensity of the illumination at the face 60 is not held exactly constant, so that the magnitude of the voltage appearing at the output of the amplifier actually varies in a corresponding way. Changes in this amplitude are indicated by the galvanorneter 68, which indicates changes in the posi tion of the spot X on the tube face 42-and hence changes in position of the object 12. Since the displacement of the light spot X is proportional to the voltage impressed on the cathode follower amplifier 64, it is clear that the voltage impressed on amplifier 66 is proportional to the vertical displacement of the object 12.

For best results, the image of the light spot X'is focused sharply on the object 12. To facilitate such focusing, the relative position between the-cathode ray oscilloscope 40 and the lens 56 is made adjustable, and the housing is adjustably mounted on a tripod 70.

A universal head 72 arranged at the top of the tripod 70 is provided at its upper end with a threaded stud 74 which is screwed firmly into a slide plate 76 that carries a rack gear 78. The slide plate 76 is slidably mounted in a pair of guide plates 80 which project downwardly from the base member 18 and which extend in adirection parallel to the axis Y-Y. The plates 80 are provided with grooves 82 into which the lateral edges 84 of the slide plate 76 project to facilitate movement of the housing in a direction parallel to the axis Y-Y. This movement is accomplished by means of a gear 86 carried by a shaft 88 that extends transversely of the housing and which is journaled in the side plates 80. The shaft 88 is restrained againstlateral movement by means of a pair of locking collars 90 and is provided at its outer end with a knurled knob to facilitate operation of the rack 86 and pinion 78 to facilitate the adjustment of the housing on the tripod '70. Locking means are provided by means of a pair of eccentric earns 94 carried by a shaft 96 which is rotated by a knurled knob 98.

As indicated in Fig. 5, the spot X moves up and down along a vertical line Z-Z on the face of the cathode ray tube 4-2. A masking plate 67 is mounted at the front of the tube, with a slot 65 aligned with the vertical line Z--Z so as to reduce fluctuations in illumination that would otherwise occur because of noise," that is, random fluctuations in illumination, originating in the parts of the phosphorescent screen 43 remote from line Z--Z by impingement of stray electrons thereon or for other reasons.

The cathode ray tube itself is mounted in a metal shield 100 which is supported by means of a front bracket 102 and a pair of symmetrically located side brackets 104 on a carriage plate 106 slidably engaging a pair of longitudinally extending rails 108 and 109 located at opposite sides of the housing 10. A locking bar 110, having a laterally extending finger 112 formed therein, is' adjustably supported by means of pins 114 extending into lateral slots 116 to lock the carriage plate 106 in a series of predetermined positions in the housing. To achieve this purpose, a series of notches 120, 122, and 124 are formed in the upper surface of the rail 109 adjacent the locking bar 110. When the finger 112 is located within one of the notches 120, 1 22, or 124, the carriage plate and hence the cathode ray tube 40 are locked firmly in position a predetermined distance from the lens 56'. The locking arrangement is completed by means of an edge cam 126 carried by a cam bar 130, which cam 126 chgages a cam follower wheel or other cam follower 132 secured to" the locking bar opposite the point midway between the slots 116.

The carriage plate 106 is provided with a rearwardly extending arm 134 having a depending clamp fin-ger 136 projecting downwardly through an elongated slot 38 that extends lengthwise of the base member 16. The cam bar extends rearwardly directly above the arm 134, and it too is provided with a downwardly depending clamp finger 146. The dam bar 130 is guided for. longitudinal movement in the housing by means of pins 142 that project through longitudinal slots 144 in the cam bar. The pins 114, 114 and 142, 142 are also secured rigidly to the carriage plate 106 to constrain the' movement of the locking bar 110 and the cam bar 130 to lat-' eral and longitudinal movement respectively. A tension spring 146 is connected between the rear pin 142 and a pin 148 fixed to the cam bar 130. The spring'146 urges the cam bar 130 in a rearward direction relative to the carriage plate 106 so that the inclined cam face 166 presses the follower 132 and hence the locking bar 110 laterally in an outward direction toward the rail 109, thus firmly holding the finger 112 in a notch 120, 122, or 124 in which it has been set.

By pressing the clamp fingers together, the cam bar 130 is urged forwardly, thus relieving the pressure on the follower 132 and permitting the carriage 106 to be moved longitudinally of the housing. With this arrangement, the carriage may be moved forwardly and backwardly as desired to any selected position in which the finger 112 engages one of the notches 120, 122, or 124'. The c'arriag'e mayralso be set in intermediate positions and there held tightly by the pressure of the finger 112 onto the rail 109.

To facilitate accurate alignment of the finger 112 with one of the notches 120, 122, and 124, an indicating pointer 150 is attached to the rear end of the bar 134 and extends laterally therefrom to the side of the housing 10. The pointer 150 is arranged to move along the outer surface of one of the side walls of the housing, and the side wall is marked with indicating lines 121, 123, and 125 corresponding to the respective notches 120, 122, and 124.

It is to be noted that in Fig. 3, the cathode ray tube is located at a position corresponding to a magnification of intermediate value, while in Fig. 4 the cathode ray tube 40 is shown in a position corresponding to low sensitivity.

In practice, the positions of the notches are so established that predetermined magnification of the movement of the light spot X is produced. In this way, scale factors are provided which correspond to the amplitude of the displacement of the object 12 for a predetermined output of the galvanometer 68. Thus, for example, the notches 120, 122, and 124 may correspond to object movements of 4", 1", and A" for the same output as indicated by the galvanometer 68. For convenience, the numbers 4, l, and A are placed above the index marks 121, 123, and 125. To achieve such proportions in a particular case, a lens 56 having a focal length of 2" was employed, and the notches 120, 122, and 124 were spaced as shown in the drawing relative to the lens 56 and the face 42 of the cathode ray tube 40. Although the tubular member 52 may be made adjustably tiltable, it is better for it to be stationary so as to avoid errors in calibration that might occur if the tubular member 52 is not oriented properly.

In this embodiment of the invention, the image of the light spot X is formed at one of three different positions in front of the lens 56 corresponding to the positions of the three notches 120, 122, and 124. For best results, once the scale has been set by movement of the cathode ray oscilloscope to one of the positions corresponding to one of the indicating marks 121, 123, and 125, the entire housing is bodily moved by manipulation of the knobs 98 and 92 to bring the image into sharp focus on a distinctive part of the object 12 the move ment of which is to be studied. The object is then set into motion or permitted to move, as the case may be, and the movement is studied or measured by means of the indicator 68.

Generally speaking, the movement of the object 12 corresponding to a predetermined'output of amplifier 62 is large when the cathode ray tube 40 is near the front of the housing and is small when the cathode ray tube 40 is at the rear of the housing.

The displacement follower of this invention may be employed ,to follow rapid and irregular movement of irregularly shaped objects 12 without making any mechanical connection of the displacement detecting system and the object. By utilizing reflected light for this purpose and by utilizing other features of the invention, including the focusing adjusting means, the inclined photosensitive surface, and the slotted mask at the front of the cathode ray tube, an improved displacement follower of greater sensitivity and greater reliability has been produced.

Though the invention has been described only with reference to a single embodiment thereof, it will be understood that it may beembodied in many other forms without departing from the principles of the invention as defined by the claims which follow. More particularly, it will be understood that other methods of changing sensitivity by altering the relative spacing of the lens and the light spot may be employed; and that the photosensitive surface may be adjustably or fixedly oriented, and that many other changes may be made in the construction,

. 6 form, and mechanical, optical, and electrical arrange ments without departing from the invention.

The invention claimed is:

1. In a displacement follower in which the displacement of a moving object is detected:

A base member having a longitudinal axis;

A source of light carried by said base member;

Optical means carried by said base member for projecting light along said longitudinal axis toward said object for focusing an image of said source of light on a part of said object;

An extended photosensitive surface carried by said base member for receiving light reflected thereto from said part of said object, the normal to said photosensitive surface being inclined toward said longitudinal axis and directed toward said object;

Deflecting means for moving said source of light along a predetermined line transverse to the longitudinal axis of said base member, whereby said image is moved transverse to said longitudinal axis at the position of said object;

Control means responsive to variations in the intensity of the light received by said photosensitive surface for operating said deflecting means to move said source of light along said predetermined line whereby the image of said source follows the moving object; and

Utilization means controlled by said control means in accordance with the displacement of said object.

2. In a displacement follower in which the displacement of a moving object is A base member;

A source of light carried by said base member;

Optical means carried by said base member for projecting light toward said object for focusing an image of said source of light on a part of said object;

An extended photosensitive surface for receiving light reflected thereto from said part of said object;

Means for adjusting the relative positions of said source of light and said optical means on said base member for adjusting the effective magnification of said optical means;

Deflecting means for moving said source of light along a predetermined line, whereby said image is moved at the position of said object;

Control means responsive to variations in the intensity of the light received by said photosensitive surface for operating said deflecting means to move said source of light along said predetermined line whereby the image of said sourcefollows the moving object; and

Utilization means controlled by said control means in detected accordance with the displacement of said object.

3. In a displacement follower in which the displacement of a moving object is detected:

A base member having a longitudinal axis;

A source of light carried by said base member;

Optical means carried by said base member for projecting light along said longitudinal axis toward said object for focusing an image of said source of light on a part of said object;

An extended photosensitive surface carried by said base member for receiving light reflected thereto from said part of said object;

Means for adjusting the relative positions of said source of light and said optical means on said base member for adjusting the effective magnification of said optical means;

Means for adjustably moving said base member along said longitudinal axis toward and away from said moving object;

Defiecting means for moving said source of light along a predetermined line transverse to the longitudinal axis of said base member, whereby said image is moved transverse to said longitudinal axis at the position of saidobject;

Control means responsive to variations in the intensity light along said predetermined line whereby the image srsaid source follows the moving object; and

Utilization means controlled by said control means in accordance with the displacement of said object.

4. In a displacement follower in which the displacement of a moving object is detected:

A housing having a base member and a front wall member;

A cathode ray oscilloscope tube mounted on said base member, the face of said tube being normal to the longitudinal axis of said housing;

Means for projecting an electron beam toward the face of said oscilloscope tube to produce a source of light thereon;

First tubular means projecting from said front wall member and having 'a lens mounted in the outer end thereof with the axis of the lens substantially normal to said face, said lens being adapted to project light along said longitudinal axis'toward said object for focusing an image of said source of light one part of said object;

Second tubular means projecting from said front wall member in a direction toward the extended axis of said n The outer ends of said tubular members being adjacent each other and being substantially equally spaced from said front wall member;

An end-on photo-multiplier tube mounted within said second tubular member, said photo-multiplier tube having a photosensitive surface adjacent the outer end of said second tubular member, said surface being transverse to the axis of said second'tubular member and being adapted to receive light reflected thereto from said part of id obj Deflecting means for moving said electron beam to cause the source of light formed on said tube face to move along a predetermined line on said face, whereby said image is moved transverse to said longitudinal axis at the position of said object;

' Control means responsive to variations in the intensity of the'light received by said photosensitive surface for operating said deflecting means to deflect said electron beam, thereby moving said source of light along said predetermined line, whereby the image of said source follows the moving object; and

Utilization means controlled by said control means in accordance with the displacement of said object.

5. In a displacement follower in which the displacement of a moving object is detected:

A housing having a base member and a front wall member;

A cathode ray oscilloscope tube mounted on said base member, the face of said tube being normal to the longitudinal axis of said housing; 1

Means for projecting an electron beam toward the face of said oscilloscope tube to produce a source of light thereon;

First tubular means projecting from said front wall member and having a lens mounted in the outer end thereof with the axis of the lens substantially normal to said face, said lens being adapted to project light along said longitudinal axis toward said object for focusing an image of said source of light on a part of said object;

Second tubular means projecting from said front wall member .in a direction toward the extended axis of said lens,

The outer ends of said tubular members being adjacent each other and being substantially equally spaced from saidfront wall member;

An end-on photo-multiplier tube mounted within said second tubular member, said photo-multiplier tube having a photosensitive surface adjacent the outer end of said second tubular member, said surface being transverse to the axis of said secondtubular member and being adapted to receive light reflected thereto from said part of said object;

Means for adjusting the relative positions of said cathode ray oscilloscope tube and said'lens'on said base member for adjusting the effective magnification of said lens;

Deflecting means for moving said electron beam to cause thesource of light formed on said tube face to move along a predetermined line on said face, whereby said image moved transverse to said longitudinal axis at the position of said object;

Control means responsive to variations in the intensity of the light received by said photosensitive surface for operating said deflecting means to deflect said electron beam, thereby moving said source of light along said predetermined line, whereby the image of said source follows the moving object; and Z Utilization means controlled by said control means in accordance with the displacement of said object.

6. In adispla cement follower in which the displacement of a moving object is detected: 7

A housing having a base member and a front wall member; i

A cathode ray oscilloscope tube mounted on said base member, the face of said tube being normal to the long'i' tudinal axis of said housing; 7

Means for projecting an electron beam toward the face of said oscilloscope tube to produce a source of light thereon;

First tubular means projecting from said front wall member and having a lens mounted in the outer end thereof with the axis of the lens substantially normal to said face, said lens being adapted to project light along said longitudinal axis toward said object for focusing an image of said source of light on a part of said objectf Second tubular means projecting from said front wall member in a direction toward the extended axis of said lens, j

The outer ends of said tubular members being adjacent each other and being substantially equally spaced from said front wall member;

An end-on photo-multiplier tube mounted within said second tubular member, said photo-multiplier tube having a photosensitive surface adjacent the outer end of said second tubular member, said surface being transverse to the axis of said second tubular member and being adapted to receive light reflected thereto from said part of said object;

Means for adjusting the relative positions of said cathode ray oscilloscope tube and said lens on said base member for adjusting the effective magnification of said lens;

Means for adjustably moving said housing along said longitudinal axis toward and" away fromsaid moving object;

Deflecting means for moving said electron beam to cause the source of lightformed on said tube face to move along a predetermined line on said face, whereby said image is moved transverse to said longitudinal axis at the position of said object;

Control means responsive to variations in the intensity of the light received by said photosensitive surface for operating said deflecting means to deflect said electron beam, thereby moving said source of light along said predetermined line, whereby the imageof said source follows the moving object; and

Utilization means controlled by said control means in accordance with the displacementcof said object.

7. In a displacement follower in which the'displaccment of wafmoving object is'detected:

A housing having a'base member and a front wall member;

A cathode ray oscilloscope tube mounted on said base member, the face of said tube being normal to the longitudinal axis of said housing; i

Means for projecting an electron beam toward the face of said oscilloscope tube to produce a source of light thereon;

First tubular means projecting from said front wall member and having a lens mounted in the outer end thereof with the ax s of the lens substantially normal to said face, said lens being adapted to project light along said longitudinal axis toward said object for focusing an image of said source of light on a part of said object;

Second tubular means projecting from said front wall member;

An end-on photo-multiplier tube mounted within said second tubular member, said photo-multiplier tube having a photosensitive surface adjacent the outer end of said second tubular member, said surface being transverse to the axis of said second tubular member and being adapted to receive light reflected thereto from said part of said object;

Means for adjusting the relative positions of said cathode ray oscilloscope tube and said lens on said base member for adjusting the efiective magnification of said lens;

Means for adjustably moving said housing along said longitudinal axis toward and away from said moving object;

Deflecting means for moving said electron beam to cause the source of light formed on said tube face to move along a predetermined line on said face, whereby said image is moved transverse to said longitudinal axis at the position of said object;

Control means responsive to variations in the intensity of the light received by said photosensitive surface for operating said deflecting means to deflect said electron beam, thereby moving said source of light along said predetermined line, whereby the image of said source follows the moving object; and

Utilization means controlled by said control means in accordance with the displacement of said object.

8. In adisplacement follower in which the displacement of a moving object is detected:

A housing having a base member and a front wall memher;

A cathode ray oscilloscope tube mounted on said base member, the face of said tube being normal to the longitudinal axis of said housing;

Means for projecting an electron beam toward the face of said oscilloscope tube to produce a source of light thereon;

First tubular means projecting from said front wall member and having a lens mounted in the outer end thereof with the axis of the lens substantially normal to said face, said lens being adapted to project light along said longitudinal axis toward said object for focusing an image of said source of light on a part of said object;

Second tubular means projecting from said front wall member;

An end-on photo-multiplier tube mounted within said second tubular member, said photo-multiplier tube having a photosensitive surface adjacent the outer end of said second tubular member, said surface being transverse to the axis of said second tubular member and being adapted to receive light reflected thereto from said part of said object;

Deflecting means for moving said electron beam to cause the source of light formed on said tube face to move along a predetermined line on said face, whereby said image is moved transverse to said longitudinal axis at the position of said object;

A mask located between said tube face and said lens for preventing light emitted from said tube face at positions remote from said predetermined line from passing through said lens;

Control means responsive to variations in the intensity of the light received by said photosensitive surface for operating said deflecting means to deflect said electron beam, thereby moving said source of light along said predetermined line, whereby the image of said source follows the moving object; and

Utilization means controlled by said control means in accordance with the displacement of said object.

9. In a displacement follower in which the displacement of a moving object is detected:

A housing having a base member and a front wall member;

A cathode ray oscilloscope tube mounted. on said base member, the face of said tube being normal to the longitudinal axis of said housing;

Means for projecting an electron beam toward the face of said oscilloscope tube to produce a source of light thereon;

First tubular means projecting from said front wall member and having a lens mounted in the outer end thereof with the axis of the lens substantially normal to said face, said lens being adapted to project light along said longitudinal axis toward said object for focusing an image of said source of light on a part of said object;

Second tubular means projecting from said front wall member in a direction toward the extended axis of said lens;

An end-on photo-multiplier tube mounted within said second tubular member, said photo-multiplier tube having a photosensitive surface adjacent the outer end of said second tubular member, said surface being transverse to the axis of said second tubular member and being adapted to receive light reflected thereto from said part of said object;

Deflecting means for moving said electron beam to cause the source of light formed on said tube face to move along a predetermined line on said face, whereby said image is moved transverse to said longitudinal axis at the position of said object;

A mask located between said tube face and said lens for preventing light emitted from said tube face at positions remote from said predetermined line from passing through said lens;

Control means responsive to variations in the intensity of the light received by said photosensitive surface for operating said deflecting means to deflect said electron beam, thereby moving said source of light along said predetermined line, whereby the image of said source follows the moving object; and

Utilization means controlled by said control means in accordance with the displacement of said object.

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